Modern automotive vehicles typically have hydraulically actuated brakes on both the front and rear wheels of the vehicle. In vehicle disc brake systems, the hub of the vehicle wheel is mounted to an axially concentric, circular disc formed of a thermally conductive and wear resistant metal. A brake caliper, fixed to the vehicle, fits around a sector of the circular disc. When a vehicle operator steps on the brake pedal, hydraulic fluid is pressurized in a brake hose connected to the brake caliper and forces friction material pads of the brake caliper against both sides of the rotating wheel disc. The frictional engagement between the caliper pads and the rotating disc serves to slow, and possibly stop, the vehicle wheel. In drum brake systems, the vehicle wheel has an axially concentric, circular metal drum surface of thermally conductive and wear resistant metal. When braking is called for, pressurized hydraulic fluid in a brake hose forces arcuate brake linings of suitable friction material outwardly against the wheel drum, to again slow, and possibly stop, the vehicle wheel.
For styling, and to control the dispersion of sand, mud, liquids, and other road spray picked up by the rotating tire, vehicle wheels are generally partially enclosed within the vehicle body within a wheel well. The wheel well is a generally circular, partially closed cavity, open at its underside and at a vehicle fender or quarter panel and extending part-way into the vehicle body. Contained within the wheel well will be the wheel, brake assembly and, often, some suspension components such as springs and shock absorbers. The wheel well is sized to accommodate the wheel and tire in all configurations which they may adopt and so its design admits of the expected range of tire movements. These may include the suspension travel and, for the front wheels, the expected range of angular inclinations on turning the steering wheel. Commonly the wheel well will be generally closed at the vehicle interior and around an appreciable portion of the tire circumference.
Generally air flow around a moving vehicle contributes significantly to the cooling of brake disc and brake drum surfaces when they are heated by the repeated wheel braking actions of normal driving. This airflow is usually more than sufficient to cool brake discs, drums, and friction materials under most commonly-experienced driving conditions, although some extra operator care might be required when towing a trailer or when driving in mountainous regions with long, steep grades. However, vehicle hood, roof, rear deck, and side surfaces are being designed with greater emphasis on reducing vehicle drag. Of course, some air flow is admitted under hood and into the engine compartment for air flow through the radiator for engine cooling. This air may also cool the front wheel brakes as it flows out of the engine compartment and under the moving vehicle. An appreciable portion, of the drag experienced by a vehicle may arise from the air flowing under the floor of the vehicle passenger compartment and interacting with the moving wheels. In many vehicles this may contribute up to about 30% or more of the total drag. Hence, more attention has been paid to smoothing underfloor air flow and minimizing interaction of the airflow with the wheels. Many vehicles are fitted with an air dam or air flow director below the front bumper for directing air flow under a moving vehicle. This improved underfloor air flow and redirection of airflow from the wheels sometimes reduces the flow of cooling air against frictionally heated brake member surfaces.
There is therefore a need in some vehicle applications for a vehicle brake cooling system which is compatible with vehicle designs which promote a low vehicle drag coefficient.